Why do you call yourself "I"?

It was a hot day, a sweat drops on my neck, grabbing a nearby cloth to wipe it out. A voice call out to me and ask "Where is my book?" and I replied "I don't know", which made me think
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Why did I just call myself " I " ?
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Out of nowhere I began to ask myself I got a name but why would I still use a word I; to tell it precisely it's just an alphabet. It made me more curious. Is it just history that we people use word "I" for definition of an individual. If so if you read "I" it should pronounce like = ai why not spell it like that?

I saw a movie in disney channel called George in the Jungle. I watched it and notice George call himself George in this sentences. "George is going play with friends" something like that. Which made me laugh, because it sounds very WRONG XDDD, or not really good.

Now thinking back, I much perhaps is a good and simple way to call yourself. If you got a two name or 3 name with many alphabets, it's definitely XD not good to hear always.
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I heard a story when I was young ( don't know if it's true or not -.-, or just and old story).....
A woman in a near by river had a 2 daughters, which are twins but left alone by the man who was the father of the children. She named the first one Ai and second one Ayi. When they are growing, there mother called them Ai and Ayi, but when they started to know some friends, Ai and Ayi we're twins and sometimes change there position (switch or trade places). Ayi was called Ai and Ai was called Ayi. But Ai didn't knew that Ayi's friends called her Ay.

pronounce Ai = I
pronounce Ay = I

Ai was confused if she's being talk too or her sister. One night, she told her twin sister not to switch places ever again. Ayi was curious why her sister Ai don't want to trade places anymore which Ayi respected and accepted. They grew closer and closer when they're getting old. But a cold rainy night, the river water is rising and their house are in danger to be caught by river flow. Ai and Ayi helped her mother to strengthen the house. It's dark and that time there are no electricity. The flow of water was too strong. Their house was destroyed and the 3 of them were carried away with the water flow.

Next morning other people saw Ai and Ayi's mother on a near by a forest beside the river. Ai and Ayi was never found. Rumors they're dead or was captured by Earthling creatures.
Ai and Ayi's mother felt sad and verge of tears hearing the news. But she didn't stop looking for them. She never found them after a 7days. But she found a trace of two bodies facing each other back lying on a ground. She had a feeling it was her daughters bodies, but decided to let them go and never decide to go for a search.

Then after years have passed people call themselves " I " for short name call, which they say it represents yourself and no one else can even switch or trade it.

(NOTE: this is just a fiction, it's never been proven and I only heard it when i was about 7 years old)